doc: mention make version in install.html

Fixes #1531.

R=adg
CC=golang-dev
https://golang.org/cl/4442088

doc/install.html

@@ -80,7 +80,7 @@ To build it, you need these programs installed:
 <li>GCC, 
 <li>the standard C libraries, 
 <li>the parser generator Bison,
-<li><tt>make</tt>, 
+<li>GNU <tt>make</tt> (version 3.81 or later),
 <li><tt>awk</tt>, and 
 <li>the text editor <tt>ed</tt>.
 </ul>